1. Field of the Invention
The present invention relates to a preparation method of crystallized sugar from an aqueous solution containing one or several sugars, multivalent cations, monovalent metal cations, monovalent anions and multivalent inorganic anions and/or organic acid anions, with prior purification of this aqueous solution by nanofiltration.
2. Description of the Related Art
The purification by demineralization of liquid products (such as a glucose syrup, sweet juices or whey) by means of ion-exchange resins has been known for many years.
The principle of such a demineralization is to make such liquid product percolate through a cationic resin and an anionic resin, the counter-ion of the former being the H+ ion and the counter-ion of the latter, the OH− ion.
While passing through the cationic resin, the cations of the liquid product are exchanged with the H+ ions of the resin, and while passing through the anionic resin, the anions of the product are exchanged with the OH− ions of this resin, the H+ and OH− ions thus released from said resins combining together to give water.
The regeneration of the resins thus used is performed by passage of an acid on the cationic resin and of a base on the anionic resin, and according to the regeneration outputs, the regeneration effluents can contain up to 2 to 3 times the inorganic load extracted from the treated liquid product.
Such highly saline effluents constitute indisputably a source of pollution.
In short, the demineralization of liquids containing inorganics by means of ion-exchange resins calls for the use of an acid and of a base for their regeneration. Besides the cost of these chemicals, this demineralization procedure produces pollutant saline effluents, and of which the processing is onerous.
Moreover, the nanofiltration technique is generally used as a pre-concentration means of aqueous solutions containing inorganics. The monovalent ions of these inorganics migrate through the nanofiltration membrane and therefore most of them are found in the permeate, whereas their multivalent ions are retained by this membrane and most of them are found concentrated in the retentate; a purification effect by demineralization is thus obtained, which remains however insufficient.